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“Living near loud traffic increases your risk of having a heart attack,” the Daily Mail today reported. The Mail added that the link “could be due to noise causing stress and sleep disturbances”. However, donning earplugs at night isn’t necessarily the answer to preventing heart attacks.

The Mail’s headline is based on a large urban Danish study showing that for every 10dB increase in the volume of road traffic noise exposure there was a 12 per cent increase in the risk of having a heart attack. This was the case for estimated traffic noise at the time of the heart attack and over the preceding five years.

This does not mean that traffic noise alone causes heart attacks. The authors speculated that higher noise exposure may increase stress and sleep disturbances, and that these in turn may lead to more heart attacks.

Similarly, the group that went on to have heart attacks was significantly less healthy than those who didn’t. While the researchers tried to adjust for this, there is still a chance that the association with road noise is just a coincidence.

This is an intriguing study. It highlights an association between traffic noise and the risk of heart attack, but it has not established cause and effect. The effect of traffic noise on the heart may be influenced by sleep disturbances or associated changes in lifestyle habits such as smoking, but these theories remain unproven and require further study.

Where did the story come from?

The study was carried out by researchers from universities, societies and other research institutions based in Denmark and The Netherlands. It was funded by the Danish Environmental Protection Agency, the Research Centre for Environmental Health, Danish Ministry of the Interior and Health, and the Danish Cancer Society.

The study was published in the peer-reviewed science journal PloS One where the article is freely available online.

What kind of research was this?

This was a large prospective cohort study that aimed to investigate the association between residential exposure to road traffic noise and the risk for new cases of heart attack that were not associated with air pollution and other known risk factors for heart attacks.

The study authors stated that both road traffic noise and ambient air pollution have been associated with risk for ischaemic heart disease. However, the evidence for this comes from only a few inconsistent studies that included both exposures. Ischaemic heart disease is characterised by a reduced blood supply to heart muscle that can cause symptoms of chest pain (angina) and decreased exercise tolerance.

The media reporting of this story was balanced, with coverage that included quotes from researchers suggesting that the relationship between traffic noise and heart attacks may be influenced by sleep disturbances. However, this is a theory put forward to explain the results, but has not been proven by this study alone.

What did the research involve?

The study was based on a cohort of 57,053 people who lived in the two largest Danish cities (Copenhagen and Aarhus) and who were born in Denmark. The participants were aged between 50 to 64 years old and were required to have no history of cancer when they enrolled in the study, which they did between 1993 and 1997.

At enrolment all the participants answered questions about food intake and lifestyle habits, including:

detailed information on present and previous smoking

physical activity

their health status, including self-reported information on diabetes and social factors

Trained staff members also measured blood pressure.

The participants were followed up until 2006 to document cases of heart attack and death that were obtained from linked medical and death records. On average people were followed up for 9.8 years, during which time researchers identified 1,600 cases of first-ever heart attack (myocardial infarction); 331 of which were fatal.

Exposure to road traffic noise and air pollution from 1998 to 2006 was estimated for all participants based on their residential address history. This estimation of noise exposure, measured in decibels (dB), used a noise-modelling programme called SoundPLAN, which the researchers said has been the standard method for noise calculation in Scandinavia for many years. This included inputting many measures of traffic including average daily traffic, vehicle distribution, travel speed, width of the road, distance of the person’s house from the road and information about the building height. However, the researchers did not measure noise levels for each participant. No information was available for "noise barriers" either – it is unclear whether the researchers meant earplugs or similar devices, or traffic-calming measures.

Associations between expected exposure to road traffic noise and new cases of heart attack were analysed to account for the influencing effects of air pollution and other potential confounders including age, sex, education, lifestyle factors, railway and airport noise.

The analysis calculated the risk of heart attack for each single year of age in the person’s life, and the average risk over the five-year period immediately before the heart attack.

What were the basic results?

The final analysis was based on 50,614 participants whose addresses were known and met the enrolment criteria. The key results were:

There was a total of 1,600 cases of heart attack; 331 of which were fatal.

Estimated residential exposure to road traffic noise was significantly associated with new cases of heart attack.

The relationship between road traffic noise and heart attack was a “linear dose-response relationship”. This means that as the amount of noise increased, the risk of heart attack also increased at a proportionate level.

For every 10dB increase in noise exposure (at the time of the heart attack) there was a 12 per cent increase in the risk of having a heart attack (incident rate ratio: 1.12, 95% confidence interval 1.02 to 1.22).

There was very similar risk increase for noise exposure over the five years immediately before the heart attack occurred (incident rate ratio 1.12, 95% confidence interval 1.02 to 1.23). The similarity is partly explained by the fact that most people hadn’t moved house in the last five years as so their traffic noise exposures were estimated to be similar in at the time of heart attack and in the previous five years.

For every 10dB increase in noise there was a trend towards an increased risk of having a fatal heart attack (incident rate ratio: 1.17, 95% confidence interval 0.96 to 1.43).

There was no significant association found between exposure to pollution and heart attack.

Those that had a heart attack were more likely to have high exposure to traffic noise and air pollution in their lives. They had also reached a lower educational level and were generally more unhealthy at enrolment, having:

smoked more

drunk more alcohol

eaten less fruit and fewer vegetables

done less physical activity

higher blood pressure

higher total cholesterol

a higher BMI

a higher prevalence of diabetes

How did the researchers interpret the results?

The researchers concluded: “Long-term residential road traffic noise was associated with a higher risk for MI [heart attack], in a dose-dependent manner.”

Conclusion

This large cohort study of Danish adults shows that people who went on to have a heart attack had a significantly higher exposure to road traffic noise in the five-year period preceding the heart attack, and that the increase in risk was proportionate to the amount of noise exposure.

This study had many strengths, including its large cohort size, prospective design, objective measures of heart attack occurrence and the relatively large number of heart attacks observed in the study period.

However, the study had some drawbacks that make it difficult to conclude firmly that experiencing traffic noise increases your risk of heart attack. These include:

Those who had heart attacks were already more unhealthy

The group that went on to have heart attacks was significantly less healthy than those who didn’t (they smoked and drank more, and did less activity). While the researchers made every effort to adjust for this fact in their statistical analysis, there is still a chance that some of the observed increase in risk associated with noise exposure is in fact due to that group having a much less healthy lifestyle in general.

Unrepresentative sample

The study authors acknowledged that the study population was not representative of the wider Danish population as the participants lived mainly in urban areas. The study findings may not be directly applicable to people living in more rural areas and other countries outside Demark.

Single outcome measure

The study only looked at the effect of noise exposure on having a heart attack. No other diseases such as stroke or cardiovascular disease were investigated.

Similarly, only traffic noise was examined, not noise from listening to loud music in headphones or work-related noise such as a noisy factory. These findings are specific to road noise and not to noise in general.

Accurately measuring noise experienced by individuals is difficult

The estimation of the noise exposure is unlikely to have been carried out perfectly. Noise was not directly measured at an individual level. Instead, estimates of expected levels were calculated from residential addresses and road traffic information. This is likely to have introduced some inaccuracy. The authors suggested that these inaccuracies are likely to be spread equally across all participants and so are unlikely to change the main study conclusion.

No information was collected on bedroom location (to estimate levels of night-time noise exposure), noise from neighbours, earplug use and hearing impairment. All of these are factors which could influence noise exposure for individuals.

Cannot prove cause and effect

The authors stated that the causal link between traffic noise and heart attack is still unclear. They did not suggest that traffic noise causes heart attacks but speculated that higher noise levels may increase stress and sleep disturbances that in turn may lead to more heart attacks.

They also hypothesised that stress and sleep disturbances can cause changes in lifestyle habits, including increased tobacco smoking, which could explain the association between traffic noise and heart attack observed in their study. Surprisingly however, the study reported finding indications of a high effect of road traffic noise on heart attacks among people who had never smoked.

This intriguing study highlighted an association between traffic noise and the risk of heart attacks. However, there is no proven causation yet established. The effect of traffic noise on the heart may be influenced by sleep disturbances or associated changes in lifestyle habits such as smoking, but these theories remain unproven and would require further study.

The best way to avoid having a heart attack is to steer clear of risk factors such as smoking and eating a salty, fatty diet. Make sure you also get plenty of exercise. It is too early to recommend moving to a quieter neighbourhood or sleeping with earplugs in.